1. Field of the Invention
The present invention relates to a semiconductor light emitting device, and in particular, to a semiconductor light emitting device using a technique in which high-power light output can be obtained with a simple structure.
2. Description of the Related Art
As broadly known, a light signal used for an optical communication system is transmitted in an optical fiber underlaid over a long distance.
Therefore, a high-power light output characteristic and high stability characteristic are required for a semiconductor laser, which is a semiconductor light emitting device used as a light source generating the light signal in the optical communication system as described above.
FIG. 12 is a perspective view for explanation of a structure of a conventional semiconductor laser 10 considered in order to obtain a high-power light output characteristic.
FIG. 13 is a cross sectional view of a main portion of the semiconductor laser 10 shown in FIG. 12.
As shown in FIG. 12, in the semiconductor laser 10, on a semiconductor substrate 11 formed from n-type InP (indium phosphor), an n-type cladding layer 12 formed from n-type InP, a first SCH (Separate Confinement Heterostructure) layer 13 formed from InGaAsP (indium gallium phosphor), an active layer 14 formed from InGaAsP, and a second SCH layer 15 formed from InGaAsP are successively formed.
Note that, the n-type cladding layer 12, the first SCH layer 13, the active layer 14, and the second SCH layer 15 are formed to be a mesa type.
A first buried layer (lower buried layer) 16 formed from p-type InP and a second buried layer (upper buried layer) 17 formed from n-type InP are formed at the both sides of the respective layers formed to be a mesa type.
A p-type cladding layer 18 formed from p-type InP is formed at the upper side of the second SCH layer 15 and the top surface of the upper buried layer 17.
A p electrode 20 is provided at the top surface of a p-type contact layer 19 formed at the top surface of the p-type cladding layer 18.
Further, an n electrode 21 is provided at the bottom surface of the semiconductor substrate 11.
As the active layer 14, a bulk structure structured from one uniform material may be used.
However, here, in order to realize a good light oscillation characteristic as the semiconductor laser 10, as shown in FIG. 13, an MQW (Multi-quantum well) structure, in which a plurality of well layers 14a and a plurality of barrier layers 14b positioned at the both sides of the respective well layers 14a are alternately formed, is used as the active layer 14.
Moreover, a multilayer structure formed from a plurality of layers 13a, 13b, and 13c is used as the first SCH layer 13 positioned at the lower side of the active layer 14 having the MQW structure.
In the same way, a multilayer structure formed from a plurality of layers 15a, 15b, and 15c is used as the second SCH layer 15 positioned at the upper side of the active layer 14.
Respective refractive indices n, with respect to the light generated by the active layer 14, of the respective layers of the n-type cladding layer 12, the first SCH layer 13 formed from the plurality of layers 13a, 13b, and 13c, the active layer 14 having the MQW structure in which the plurality of well layers 14a and the plurality of barrier layers 14b are included, the second SCH layer 15 formed from the plurality of layers 15a, 15b, and 15c, and the p-type cladding layer 18, are set so as to be the characteristics of the refractive indices as shown in FIG. 14.
Namely, the refractive index of the active layer 14 at the center is set to the highest, and the refractive indices of the respective cladding layers 12, 18 at the both sides are set so as to be equal to one another and to the lowest amount those of the layers.
Then, the refractive indices of the plurality of layers 13a, 13b, 13c, and 15a, 15b, 15c of the first SCH layer 13 and the second SCH layer 15 are respectively set so as to be gradually lower.
In this way, the refractive indices as the entire semiconductor laser 10 are set so as to have the characteristic of the refractive indices which is vertically symmetrical (FIG. 14) with respect to the active layer 14 serving as the center.
When a predetermined direct voltage is applied between the p electrode 20 and the n electrode 21 of the semiconductor laser 10 having such a characteristic of the refractive indices, light P having power corresponding to the current region is thereby generated at the active layer 14.
Further, the light P generated at the active layer 14 is emitted to the exterior from both end surfaces (facet) 22a and 22b of the semiconductor laser 10 shown in FIG. 12.
Note that, in the semiconductor laser 10, due to the refractive index of the active layer 14 being set to be higher than the refractive indices of the respective cladding layers 12 and 18, other than the fact that some of the light P generated at the active layer 14 are leaked to the respective cladding layers 12 and 18, an optical waveguide path for preventing dissipation is formed.
In accordance therewith, it is anticipated that the semiconductor laser 10 in which high-power light output can be obtained at a high current region is realized.
However, in the semiconductor laser 10, because the characteristic of the refractive indices is vertically symmetrical with respect to the active layer 14 serving as the center, the distribution of the light P generated at the active layer 14 is vertically symmetrical with respect to the active layer 14 serving as the center.
Therefore, the distributions of the light P leaked to both the cladding layers 12 and 18 are the same, and the quantity of optical loss by intervalence band absorption on the basis of the distribution of light in the p-type cladding layer 18 cannot be avoided, and the light output as the semiconductor laser 10 is reduced by the quantity of optical loss.
Further, because the electrical resistance of the p-type cladding layer 18 is relatively high, the heating value by the p-type cladding layer 18 at a high current region is made large, which means the light output as the semiconductor laser 10 is reduced.
Accordingly, it is difficult to realize the semiconductor laser 10 in which high-power light output can be obtained at a high current region.
Note that, in order to make the light output of the semiconductor laser 10 having such a structure have much higher power, the first SCH layer 13 and the second SCH layer 15 which respectively have intermediate refractive indices are intervened between the active layer 14 and both the cladding layers 12, 18.
Namely, in accordance therewith, the carriers which are injected can be concentrated in the vicinity of the active layer 14, and at this time, because the carriers and light are simultaneously concentrated at the same region in the vicinity of the active layer 14, the luminous efficiency as the semiconductor laser 10 is high.
Further, in order to make the light output of the semiconductor laser 10 having such a structure have high power, it is effective that an attempt is made to reduce the optical confinement coefficient to the first SCH layer 13, the second SCH layer 15, and the active layer 14.
However, when the optical confinement coefficient to the first SCH layer 13, the second SCH layer 15, and the active layer 14 are lowered, due to the components of the light passing through both the cladding layers 12 and 18 increasing, another problem arises.
In other words, in accordance with the fact that the components of the light passing through the both cladding layers 12 and 18 increases, it is necessary to increase the thickness of both the cladding layers 12 and 18.
However, at the p-type cladding layer 18, as described above, because the electrical resistance is relatively high, the electrical resistance of the entire element is increased due to the increase of the p-type cladding layer 18, and the heating value of the element at the high-current region is made large, making it difficult to make the light output of the semiconductor laser 10 have much higher power.
Moreover, if the distribution of light in the p-type cladding layer 18 among both the cladding layers 12 and 18 is increased, the quantity of optical loss by intervalence band light absorption described above increases.
The increase of the quantity of optical loss by intervalence band light absorption can be prevented due to the p-type impurity concentration of the p-type cladding layer 18 being made small.
However, if the p-type impurity concentration of the p-type cladding layer 18 is made small, due to the electrical resistance of the entire element including the p-type cladding layer 18 further increasing, high-power light output cannot be obtained as the semiconductor laser 10.
As a method for solving the problem of the optical loss by intervalence band light absorption, as shown in FIG. 15, a technique in which, due to an optical field control layer 23 having a refractive index which is higher than the refractive index of the n-type cladding layer 12 and is close to the refractive index of the active layer 14 being provided in the n-type cladding layer 12, the distribution of light is shifted to the n-type cladding layer 12 side, and the quantity of light distributed in the p-type cladding layer 18 is reduced, is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-174394 which is Patent Document.
However, in this way, if the optical field control layer 23 having the refractive index close to the refractive index of the active layer 14 is provided in the n-type cladding layer 12, not only is the structure complicated, but also a new problem arises.
Namely, because the optical field control layer 23 as described above has the same structure as that of the active layer 14, when the optical field control layer 23 is provided at a position far away from the first SCH layer 13, another optical waveguide path is formed, and due to the distribution of light being made to be the double-humped characteristic, the operation as the semiconductor laser 10 is made unstable.
Accordingly, the optical field control layer 23 must be provided in the vicinity of the first SCH layer 13.
However, if the optical field control layer 23 whose refractive index is high is provided in the vicinity of the first SCH layer 13, due to the equivalent refractive indices of the entire waveguide path being made high, an oscillation mode of the semiconductor laser 10 is easily displaced from a desired single mode to a lateral high-order mode.
Further, the displacement to the lateral high-order mode can be prevented by making the width of the region including the active layer 14, the first SCH layer 13, and the second SCH layer 15 narrow.
However, if the width of the region including the active layer 14, the first SCH layer 13, and the second SCH layer 15 is made narrow, the increases of the electrical resistance and the thermal resistance of the entire element are bought about, and the luminous efficiency of the semiconductor laser 10 is more decreased.